About Heart & Blood
The cardiovascular system includes the heart, blood, and a huge network of blood vessels. The heart acts as a muscular pump, while blood carries oxygen, nutrients, hormones, heat, immune cells, and waste products between tissues.
Four Chambers
The human heart has two upper chambers and two lower chambers.
Learn MoreValves
Heart valves help keep blood moving in the correct direction.
Learn MoreArteries, Veins, and Capillaries
Different blood vessels are built for different parts of circulation.
Learn MoreRed Blood Cells
Red blood cells carry most of the oxygen in the blood.
Learn MoreWhite Blood Cells
White blood cells are important parts of the immune system.
Learn MorePlatelets and Clotting
Platelets help seal damaged blood vessels.
Learn MorePulse
A pulse is a pressure wave produced when the heart pumps blood into arteries.
Learn MoreTwo Main Circuits
Blood follows a pulmonary circuit and a systemic circuit.
Learn MoreFour Chambers
The right side receives oxygen-poor blood returning from the body and pumps it toward the lungs. The left side receives oxygen-rich blood from the lungs and pumps it through the aorta to the rest of the body.
Valves
Four main valves open when pressure pushes blood forward and close when pressure begins to reverse. The familiar heart sounds heard with a stethoscope are closely related to valve closure and blood movement.
Arteries, Veins, and Capillaries
Arteries carry blood away from the heart under relatively high pressure. Veins return blood to the heart. Tiny capillaries have very thin walls that allow oxygen, carbon dioxide, nutrients, and wastes to move between blood and tissues.
Red Blood Cells
They are packed with hemoglobin, a protein containing iron that binds oxygen. Mature human red blood cells lack a nucleus, leaving more internal space for hemoglobin.
White Blood Cells
Several kinds circulate in blood, including neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Their jobs include attacking microbes, coordinating immune responses, removing damaged material, and producing antibodies.
Platelets and Clotting
When a vessel is injured, platelets stick to the area and help form a temporary plug. A chain of clotting reactions then produces fibrin fibers that strengthen the developing clot.
Pulse
You can feel a pulse where an artery lies close to the skin. Heart rate changes with activity, emotion, temperature, sleep, fitness, medicines, and many other factors.
Two Main Circuits
The pulmonary circuit moves blood between the heart and lungs for gas exchange. The systemic circuit carries blood between the heart and the rest of the body. These two loops operate continuously together.